1. Field of the Invention
This invention relates generally to nucleic acid sequences encoding polypeptides that are associated with abiotic stress responses and abiotic stress tolerance in plants. In particular, this invention relates to nucleic acid sequences encoding polypeptides that confer drought, cold, and/or salt tolerance to plants.
2. Background Art
Abiotic environmental stresses, such as drought stress, salinity stress, heat stress, and cold stress, are major limiting factors of plant growth and productivity. Crop losses and crop yield losses of major crops such as soybean, rice, maize (corn), cotton, and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries.
Plants are typically exposed during their life cycle to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on development, growth, and yield of most crop plants are profound. Continuous exposure to drought conditions causes major alterations in the plant metabolism, which ultimately lead to cell death and consequently yield losses.
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies to develop new lines of plants that exhibit resistance (tolerance) to these types of stresses are relatively slow and require specific resistant lines for crossing with the desired line. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to drought, cold and salt tolerance in model, drought- and/or salt-tolerant plants are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways. This multi-component nature of stress tolerance has not only made breeding for tolerance largely unsuccessfull, but has also limited the ability to genetically engineer stress tolerance plants using biotechnological methods.
Drought and cold stresses, as well as salt stresses, have a common theme important for plant growth, and that is water availability. Plants are exposed during their entire life cycle to conditions of reduced environmental water content, and most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Further-more, most of the crop plants are very susceptible to higher salt concentrations in the soil. Because high salt content in some soils results in less water being available for cell intake, high salt concentration has an effect on plants similar to the effect of drought on plants. Additionally, under freezing temperatures, plant cells lose water as a result of ice formation that starts in the apoplast and withdraws water from the symplast. A plant's molecular response mechanisms to each of these stress conditions are common, and protein phosphatases play an essential role in these molecular mechanisms.
It is well recognized that reversible phosphorylation of proteins controls many cellular processes in plants and animals. The phosphorylation status of proteins is regulated by the opposing activities of protein kinases and protein phosphatases. Phosphorylation of eukaryotic proteins occurs predominantly on serine and threonine residues, and to a lesser extent, on tyrosine residues. In animals, protein phosphorylation plays well-known roles in diverse cellular processes such as glycogen metabolism, cell cycle control, and signal transduction (Smith, R. D. and Walker, J. C., 1996, Annu. Rev. Plant Physiol. Plant Mol. Biol. 47:101–125).
Protein phosphatase activities have been reported in most plant subcellular compartments, including mitochondria, chloroplast, nuclei and the cytosol, and are associated with various membrane and particulate fractions. Some protein phosphatases are poorly characterized and may represent novel enzymes that are unique to plants. Others have biochemical properties that are very similar to well-known mammalian protein phosphatases, such as cytosolic protein serine/threonine phosphatases (MacKintosh C. and Cohen P. 1989 Biochem. J. 262:335–339). Two such plant serine/threonine phosphatases have been identified that function similar to mammalian type-1 (PP1) and type-2 (PP2) protein serine/threonine phosphatases. Biochemical and genetic studies in plants implicate PP1 and/or PP2 activity in signal transduction, hormonal regulation, mitosis, and control of carbon and nitrogen metabolism (Smith, R. D. and Walker, J. C., 1996, Annu. Rev. Plant Physiol. Plant Mol. Biol. 47:101–125).
Experimental evidence has implicated the involvement of protein phosphatases in the plant stress-signaling cascade, and more particularly, in stress perception and signal transduction linked to physiological mechanisms of adaptation in plants. For example, protein phosphatase 2C (PP2C) has been shown to be involved in stress responses in plants (Sheen, J. 1998 Proc. Natl. Acad. Sci. USA 95:975–980). It has also been demonstrated that, in yeast, the PP2B phosphatase calcineurin (CaN) is a focal component of a Ca2+-dependent signal transduction pathway that mediates Na+, Li−, and Mn2+ tolerance of Saccharomyces cerevisiae (Cunningham, K. W. and Fink, G. R. 1996 Mol. Cell. Biol. 16:2226–2237). CaN functions to limit intracellular Na+ accumulation by regulating processes that restrict influx and enhance efflux of this cation across the plasma membrane. CaN also participates in cytosolic Ca2+ homeostasis through the positive regulation of Golgi apparatus and vacuolar membrane-localized P-type ion pumps and negative control of a vacuolar H+/Ca2+ exchanger. Interestingly, overexpression of yeast CaN confers salt tolerance in plants, strongly indicating that modulation of stress signaling pathways by expression of an activated protein phosphatase substantially enhances plant stress tolerance (Pardo, J. M. et al. 1998 Proc. Natl. Acad. Sci. USA 95:9681–9686).
Although some genes that are involved in stress responses in plants have been characterized, the characterization and cloning of plant genes that confer stress tolerance remains largely incomplete and fragmented. For example, certain studies have indicated that drought and salt stress in some plants may be due to additive gene effects, in contrast to other research that indicates specific genes are transcriptionally activated in vegetative tissue of plants under osmotic stress conditions. Although it is generally assumed that stress-induced proteins have a role in tolerance, direct evidence is still lacking, and the functions of many stress-responsive genes are unknown.
There is a need, therefore, to identify genes expressed in stress tolerant plants that have the capacity to confer stress resistance to its host plant and to other plant species. Newly generated stress tolerant plants will have many advantages, such as increasing the range in which crop plants can be cultivated by, for example, decreasing the water requirements of a plant species.